(1) Field of the Invention
This invention relates to a method and apparatus for generating exposure data and, more particularly, to a method and apparatus for generating exposure data for transferring a pattern by exposing a semiconductor substrate or a reticle by the use of a plurality of charged particle beams.
(2) Description of the Related Art
The process for fabricating semiconductor devices, such as large scale integrated circuits (LSIs), includes a lithography process for transferring a circuit pattern (pattern) onto a semiconductor substrate (wafer) by exposure.
In recent years semiconductor devices have become minuter. As a result, higher resolution is needed in the lithography process, so techniques using a charged particle beam, such as an electron beam or an ion beam, are examined and put to practical use. For example, the beam diameter of an electron beam can be narrowed down to several nanometers. Therefore, minute patterns with line width narrower than or equal to 100 nm can be formed.
In the early lithography processes using an electron beam, writing was performed by scanning a narrowed electron beam. That is to say, what is called a single stroke writing method was adopted. With this method, however, it takes a long time to form a large pattern, so exposure throughput is low. Therefore, this method was used only for making a reticle for optical exposure, fabricating a semiconductor device having a minute pattern which is difficult to form by optical exposure on an experimental basis, or the like. As a result, the following lithography techniques are developed in succession.
FIGS. 11A and 11B are views showing examples of a lithography technique using an electron beam. FIG. 11A is a view showing a lithography technique called a variable rectangular exposure method. FIG. 11B is a view showing a lithography technique called a cell projection exposure method.
In the variable rectangular exposure method shown in FIG. 11A, an electron beam emitted from an electron gun 800 is shaped into a variable rectangle by a first aperture 801 having an opening 801a. In addition, the electron beam is shaped into a rectangle or a triangle of a desired size by a mask 802 having an opening 802a. A resist with which a wafer (or reticle) 803 is coated is irradiated with the shaped electron beam to form a pattern 804. With this method, however, the pattern to be formed is divided into small blocks and one shot of exposure is performed for each small block. Accordingly, exposure throughput is low. This problem can be solved by using the cell projection exposure method shown in FIG. 11B.
In the cell projection exposure method, a mask 805 on which a plurality of opening patterns 805a are formed is used. In many semiconductor devices, the same pattern or the same pattern group is used repeatedly. An opening having the shape of such a pattern or pattern group repeatedly used is used as each opening pattern 805a. As a result, a unit pattern 806 can be formed in block on the wafer (or reticle) 803 with a single shot. This enhances exposure throughput. The variable rectangular exposure method shown in FIG. 11A is used for forming a pattern which is not used repeatedly.
In order to obtain high throughput in the cell projection exposure method, it is necessary to extract a pattern or a pattern group used many times, that is to say, data highly effective in performing cell projection exposure from layout data for exposure and to generate exposure data in which the extracted data is used as blocks for cell projection exposure. Accordingly, a technique for extracting blocks for cell projection exposure from layout data for exposure including a plurality of patterns so as to minimize the total number of shots of an electron beam for exposure data for one layer by which a pattern is transferred onto a wafer is disclosed (see, for example, Japanese Unexamined Patent Publication No. Hei5-182899).
FIG. 12 is a sectional view showing the rough structure of a conventional electron beam exposure apparatus.
In an electron beam exposure apparatus 810 shown in FIG. 12, an electron beam emitted from an electron gun 811 is shaped and deflected by an electron lens 812, an aperture 813, an electron lens 814, and a deflector 815 and is applied to a predetermined position on a mask 817 via an electron lens 816. The mask 817 is held by a mask stage 818. In addition, the electron beam applied to the predetermined position on the mask 817 having an opening in the shape of a pattern is deflected by a deflector 819 and an electron lens 820 and is applied to a predetermined position on a wafer (or reticle) 821. As a result, the pattern is formed.
Meanwhile, a multi-beam exposure apparatus in which a plurality of electron beams are applied to one wafer or reticle at one time is proposed. A combination of this multi-beam exposure apparatus and the cell projection exposure method is expected to significantly enhance exposure throughput.
FIGS. 13A and 13B are schematic views of multi-beam exposure methods. FIG. 13A is a schematic view of a multi-beam exposure method using a mask on which blocks for cell projection exposure are formed. FIG. 13B is a schematic view of a multi-beam exposure method in which blocks for cell projection exposure are represented by electronic data.
With the multi-beam exposure method shown in FIG. 13A, electron beams emitted from a plurality of electron guns 831 are shaped and deflected by apertures 832 and deflecting sections (each including the electron lens, the deflector, and the like shown in FIG. 12) 833 and are applied simultaneously to opening patterns, being blocks for cell projection exposure, at predetermined positions on masks 834a. In addition, the electron beams are deflected by deflecting sections 835 and are applied to predetermined positions on a wafer (or reticle) 836 to simultaneously form a plurality of patterns 837.
Meanwhile, with the multi-beam exposure method shown in FIG. 13B, a physical mask is not used. In this case, blocks for cell projection exposure are represented by electronic data stored in a database 840 and patterns 837 are formed by controlling the opening and closing of slits 834b. 
It is necessary to extract a pattern or a pattern group used many times, that is to say, data highly effective in performing cell projection exposure from layout data for exposure and to generate exposure data in which the extracted data is used as blocks for cell projection exposure. This applies both to a case where opening patterns, being blocks for cell projection exposure, are formed on a mask and to a case where blocks for cell projection exposure are held as electronic data.
Moreover, with the multi-beam exposure methods, a plurality of charged particle beams are applied simultaneously to corresponding areas on a wafer or a reticle. Accordingly, exposure throughput depends on time taken to perform exposure in one of these areas (unit areas) where the number of shots is the greatest.